1. Field of the Invention
The present invention relates to a gas sensor for measuring a concentration of a predetermined gas component in a measurement gas, and more particularly to a technique of retaining and fixing a sensor element thereof.
2. Description of Related Art
Conventionally, in order to know a concentration of a desired gas component in a measurement gas, there have been known various measuring apparatuses. For example, as an apparatus which measures an NOx concentration in the measurement gas such as a combustion gas or the like, there has been known a gas sensor (an NOx sensor) having an oxygen ion conductivity such as a zirconia (ZrO2) or the like (for example, refer to Japanese Patent Application Laid-Open No. 2006-284223 (patent document 1)). The sensor element of the gas sensor mentioned above typically has a plurality of electrode terminals on a surface for applying an electric voltage, picking up a detection signal, supplying an electric power to a heater part and the like.
On the other hand, the gas sensor is provided with a contact member in which the sensor element is inserted so as to retain. For example, there has already known a gas sensor having a contact member which is provided with a housing forming an insertion port in which the sensor element is inserted by a pair of housing members arranged so as to be opposed to each other, a plurality of contact point members which are additionally provided in the housing members and constructed by metal terminals, and a plurality of lead wires which are connected to the contact point members and achieve an electric continuity between the sensor element and an external portion (for example, refer to Japanese Utility Model Application Laid-Open No. 4-110972 (patent document 2)).
In the gas sensor disclosed in Japanese Utility Model Application Laid-Open No. 4-110972, the contact member retains the sensor element which is inserted in an insertion port, in a state that the contact point member is brought into contact with the electrode terminal, thereby obtaining the electric continuity between the sensor element and the external portion. In other words, in the gas sensor disclosed in Japanese Utility Model Application Laid-Open No. 4-110972, the contact point member plays a role in a contact point with the electrode terminal. Specifically, housing members are fitted into a fixing metal fixture having a C-shaped form in a cross sectional view with the pressing spring in its upper and lower sides, while the sensor element is inserted in the insertion port therebetween, and then, a caulking ring is arranged in an outer periphery of the fixing metal fixture and the pressing spring and is caulked so that the spring ring is displaced to energize the contact point member to the electrode terminal with an elastic force of the pressing spring. Thus, a retention of the sensor element and a preservation of the electric continuity are achieved.
The fixing metal fixture disclosed in Japanese Utility Model Application Laid-Open No. 4-110972 not only has a role for fixing the spring member as mentioned above, but also has a role for constraining a pair of housing members in a predetermined arrangement relationship as long as the sensor element is inserted and the caulking ring is caulked. The constraint mentioned above is necessary for correctly bringing the electrode terminal into contact with the corresponding contact point member without any displacement, at a time of retaining and fixing the sensor element to the contact member between a pair of housing members (that is, in the insertion port) by caulking the caulking ring. In the case of Japanese Utility Model Application Laid-Open No. 4-110972, by forming the fixing metal fixture in the C-shaped form in the cross section, its elasticity as a leaf spring is utilized for retaining and constraining the housing member.
In the gas sensor disclosed in Japanese Utility Model Application Laid-Open No. 4-110972, the elastic force of the pressing spring acts on the housing via the fixing metal fixture with the caulking ring being caulked, whereby the sensor element is fixed to the housing. At this time, the contact point member provided in the housing energizes the electrode terminal, whereby a contact between the electrode terminal and the contact point member is secured.
However, at that time, since the fixing metal fixture has the C-shaped form in the cross section as mentioned above, there is such a problem that a force acting on the housing comes to have an asymmetry (a nonuniformity). In the gas sensor structured such that a plurality of electrode terminals come into contact with the corresponding contact point members, the force with which each of contact point members energizes the electrode terminal differs according to its position, because of such nonuniformity. As a result, a local contact failure is likely to occur.